Apparatus and method for recirculating working medium in a forced flow steam generator

ABSTRACT

At least some of the working medium leaving the evaporator is condensed on at least some of the preheated feed water and is fed with the feed water to the evaporator inlet. The temperature of the condensate and feed water mixture is kept 10* to 20* C. below the saturated steam temperature. The condensate can be formed in a separable mixer or in a water separator.

United States Patent [72] lnvc ntor Ewald K. Burltle Stuttgart, Germany [2|] Appli No 844,576

[22] Filed July 24. I969 [45] Patented July 13, I971 [73] Assignee Sulur Brothers, Limited Wlnterthur, Switzerland [32] Priority July 25, 1968 (33] Germany [54] APPARATUS AND METHOD FOR RECIRCULATING WORKING MEDIUM IN A FORCED FLOW STEAM GENERATOR 10 Claims. 2 Drawing Figs.

[52] US. Cl 122/406-8 [51] Int. Cl v r r i r F23) 29/06 [50] Field of Search H l22/406, 406 S [56] References Cited UNITED STATES PATENTS 3,038,453 6/l962 Armacost 122/406 3.4! 1,484 M11968 Kraus 122/406 Primary Examiner-Kenneth W. Sprague Attorney-Kenyon and Kenyon Reilly Carr 8: Chapin ABSTRACT: At least some of the working medium leaving the evaporator is condensed on at least some of the preheated feed water and is fed with the feed water to the evaporator inlet. The temperature of the condensate and feed water mixture is kept l0 to 20 C. below the saturated steam temperature. The condensate can be formed in a separable mixer or in a water separator.

PATENTED JUL 1 3 l9?! SHEET 1 OF 2 lnventpr EWALD K- BLJRKLE PATENTED M13197:

SHEET 2 0F 2 Inventor: Ew/un K BURMLE 'O EEYS APPARATUS AND METHOD FOR RECIRCULATING WORKING MEDIUM IN A FORCED FLOW STEAM GENERATOR This invention relates to an apparatus and method for recirculating working medium in a forced flow steam generator. More particularly, this invention relates to an apparatus and method for recirculating working medium through an evaporator or a forced flow steam generator.

Forced flow steam generators have been known in which combustion chambers have been constructed from tubes which extend continuously vertically while being suspended since such are much simpler and cheaper with respect to design, production and assembly. Further, a tube system of this kind is essential if the tubes are to be welded together in an absolute seal tight manner. This latter requirement has been made mandatory increasingly in gas-fired and oil-fired boiler plants. Generally, the ratio of the mass flow to the heat flux density required to cool such combustion chamber tubes has been obtained by circulation of the water in the system. Another advantage of such forced flow steam generators has been the reduced power requirements of the plant itself, since the pressure drop in the evaporator of the generator fluctuates to a much lesser extent than in the case of a forced flow once through system.

However, because the wall tubes extend continuously over the entire combustion chamber height, the evaporator heating surface has been much larger than is required for the transfer of the evaporation heat. Accordingly, the evaporator tubes had to be protected from heat radiation in the top part of the combustion chamber by preceding tubes of a wall superheater. Apart from the fact that the evaporator and the wall superheater had to be insulated from one another in order to prevent any heat transfer from the wall superheater to the evaporator, and the fact that it has not been a simple matter to mount the wall superheater, screening of part of the evapora tor necessitates the use of large quantities of material which is not used for heat transfer. In addition, in the case of browncoal-fired steam generators in which the coal is dried in the mills by combustion gases drawn from the combustion chamber, the combustion gases have to be drawn from the re gion of the wall superheater, i.e. through two tube walls which have different rates of expansion.

The invention obviates the above disadvantage. To this end, the invention provides a steam generator with an evaporator in which in the top load range of the steam generator at least some of the working medium leaving the evaporator is condensed on at least some of the preheated feed water and is fed, together with the feed water, to the evaporator inlet. The forced flow steam generator for performing this function utilizes a mixer in which some of the working medium leaving the evaporator condenses on at least some of the preheated feed water and is fed together with feed water to the evaporator inlet. With this system, it is no longer necessary to provide a wall superheater in the top part of the combustion chamber, because in certain states of operation the evaporator is operated with slight superheating at its end. When the saturated steam temperature is exceeded at the evaporator outlet, according to the invention, a controllable part of the now slightly superheated steam is condensed on the preheated feed water in the mixer and the condensate is fed together with the feed water to the evaporator inlet by means of the circulating pump. If the steam temperature at the evaporator outlet drops to the saturated steam temperature when the steam generator loading drops, the steam generator is operated conventionally, i.e. the excess water required to maintain the mass flow is separated in the water separator and there is no condensation of steam on the feed water.

The method of the invention is such that the steam generator is operated so that the temperature of the condensate and feed water mixture forming as a result of the condensation of the working medium is kept at a temperature about l-2U" C. below the saturated steam temperature. This safety margin is necessary to prevent the water in the mixer from evaporating in the event of a sudden pressure drop, and to prevent steam being fed to the circulating pump.

According to one embodiment of the steam generator according to the invention, the mixer is connected to a water chamber of a water separator via a line while a bypass line equipped with a control and nonreturn valve means is provided between the evaporator outlet and the mixer to bypass the water separator. In order to ensure the uniform mixing of feed water and recycled steam in the mixer at the maximum speed, internal fittings consisting of perforate plates or the like, such as are known in steam washers, are so arranged in the mixer that the feed water and the recycled steam flow jointly through such fittings.

Since the use of a tightly sealing valve means in the bypass line has the disadvantage when the steam generator is operated without a steam recycle that the valve means must seal the bypass line from the feed water and excess water, any failure of the valve means would result in water flowing out of the mixer into the water separator or superheater where the water might cause damage. To obviate this, advantageously, under all load conditions of the steam generator, some of the working medium leaving the evaporator is fed to the mixer. The amount of such working medium only being such as to prevent any entry of feed water or recycled water into the bypass line so that the amount of working medium acts as a barrier flow.

In another embodiment, instead of disposing the mixer separately from the water separator, the mixer is combined with the water separator. With such a construction, the mixer forms part of the water separator and the line coming from the water preheater is divided into two branches, one of which leads into the separator above the water level while the other branch leads into the separator below the water level. A distributor is also provided at the point where the line branches to control the path of flow.

The invention thus provides a method of operating a forced flow steam generator which is constructed for subcritical pressure. The invention further provides a steam generator which is constructed with a water separator connected to the evaporator outlet and to the evaporator inlet so that the working medium can be recycled, at part load, via the water separa' tor and a circulating pump to the evaporator inlet. In addition, the combustion chamber walls of the steam generator are formed of parallel connected vertical evaporator tubes which extend over the entire combustion chamber height.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

.FIG. 1 illustrates a flow diagram of a forced flow steam generator utilizing a mixer separate from a separator according to the invention; and

FIG. 2 illustrates a flow diagram of a modified forced flow steam generator according to the invention.

Referring to FIG. 1, the steam generator is constructed with a preheater 2, an evaporator 6, and a superheater 1] which are interconnected in the flow path of a working medium as well as any other known components as is known. The preheater 2 is connected to a feed pump (not shown) which serves to deliver feed water to the preheater 2 via a feed water control valve l and via suitable lines to a mixer 3 at the opposite side which receives the feed water from the preheater. The mixer 3, in turn, is connected via a circulating pump 4 to the inlet 5 of the evaporator 6 in order to deliver the feed water thereto. The evaporator 6 is constructed of parallel connected vertical evaporator tubes which serve to define the walls of a combustion chamber within the steam generator by extending over the entire combustion chamber height. in addition, a water separator 8 is connected between the outlet 7 of the evaporator 6 and a line [0 which leads to the superheater ll so that water can be separated from the working medium flowing from the evaporator 6 to the superheater 11. The

water separator chamber outlet is also connected by a line 9 containing a nonreturn valve M to the mixer 3 in order to direct the separated water to the mixer 3. A suitable line hav ing a valve therein is also connected to the bottom of the water separator 8 in order to act as a drain or for other purposes as is known.

The outlet 7 of the evaporator 6 is also connected to the mixer 3 via a bypass line l2 having a control valve 13 and u nonreturn valve 14 therein. The control valve 13 functions to control a flow of steam through the line 12 to the mixer 3 while the nonreturn valve [4 prevents a return flow from the mixer 3 to the evaporator outlet 7. The mixer 3 thus serves as a means for condensing at least some of the working medium leaving the evaporator outlet 7 on at least some of the pre' heated feed water for subsequent feeding with the feed water to the evaporator inlet 5,

in order to operate the steam generator, feed water is delivered to the preheater 2 via the feed water control valve l in order to be preheated as is known. After flowing through the water preheater 2, the preheated feed water flows to the mixer 3 and from there is fed by the circulating pump 4 to the inlet 5 of the evaporator 6.

If the steam generator is operated at a temperature equal to the saturated steam temperature at the evaporator outlet 7. the mixture of steam and water at the evaporator outlet 7 is fed to the water separator 8 where the water and steam are separated. The separated water then flows through the line 9 to the mixer 3 and is fed to the evaporator inlet 5 together with the preheated feed water by means of the circulating pump 4 The steam passes from the separator 8 via the line 10 to the su' perheater H and is superheated to the required temperature and then flows to a turbine (not shown).

lfthe steam generator is operated with a steam temperature at the outlet 7 of the evaporator 6 above saturated steam tern perature, part of the generated steam is fed to the mixer 3 via a bypass line 12 with the remainder being fed to the water separator. The amount of steam fed to the mixer is controlled by the control valve 13 according to the amount of water required to be recirculated to the evaporator 6. The mixture temperature in the mixer 3 forms a limit for the amount of steam recycled, such temperature being kept between 10 and C below the saturated steam temperature. To this end, any suitable control means can be used to operate the control valve 13 in response to the amount of steam required.

Irrespective of the type of operation of the steam generator, 3 quantity of working medium is continually fed from the evaporator outlet 7 to the mixer 3 in order to act as a barrier against any return How of water from the mixer 3. Any return flow of water from the mixer 3 to the separator 8 is also prevented by the nonreturn valve 14.

The following specific example may be given in explanation of the invention. A steam generator having a pressure of about 200 atmospheres gauge at the evaporator outlet 7 will be taken as a basis. For a load from 100 percent to about 75 per cent, superheated steam at a temperature of 380-390 C leaves the evaporator 6, having been generated at a saturated steam temperature of about 365 C, the feed water tempera ture being 260 C. in this load range, part of the steam generated is fed to the mixer 3. The temperature difference between the preheated feed water and the recycled steam al' lows condensation of about percent of the steam generated without dropping below the safety margin in relation to the saturated steam temperature in the mixer 3. Consequently, the steam generator can be operated with a l.3 recirculation in the evaporator.

irrespective of whether the steam generator is operated at a variable pressure or a fixed pressure, adequate stability of the flow of working medium through the evaporator is always ensured.

It IS also immaterial whether the vertical combustion chamber tubes are welded together or whether the steam generator is constructed by the skin casing method or as a brickwork structure.

For the performance of the invention it is immaterial whether the water separator 8 and the mixer 3 are separate components of the steam generator or are combined to form a unit. For example, referring to FIG. 2, wherein like parts are designated with like reference characters as above, the line 18 coming from the water preheater 2 is divided into two branches 19, 20, the top branch [9 of which leads into the water separator 8 above the water level while the bottom branch 20 leads into the separator 8' below the water level. At the point where the line [8 branches, a distributor 2| is provided to control the apportionment of the feed water between the branches I9, 20 and is controlled in dependence on the temperature of the working medium leaving the water chamber of the separator 8. To this end, a temperature sensitive element 23 is connected in the line 22 and is connected to a controller 24, the output of which is fed to the distributor 21 in order to adjust the distributor 21 to control the paths of flow. A pressure sensitive element 25 is also connected in the line 22in order to adjust the temperature set value in response to the sensed pressure via a function transmitter 26.

When the steam generator is operated with a temperature at the evaporator outlet 7 above saturated steam temperature, part of the steam entering the separator 8' is condensed on the partial flow of feed water entering the separator above the water level via the branch 19. The remaining quantity of feed water from line 18 flows through the bottom branch 20 directly into the water chamber ofthe separator 8'. By means of the control system 23-26, the distributor 21 is so adjusted that the temperature of the working medium in the line 22 remains between it) and 20 C below the saturated steam temperature.

When the steam generator is operated with a temperature equal to the saturated steam temperature at the evaporator outlet 7, the distributor 2] is so adjusted that practically all the feed water flow line 18 enters the separator 8' via the bottom branch 20.

What claim is:

i. In a steam generator having an evaporator parallel connected evaporator tubes defining and extending over the height of a combustion chamber, said evaporator further including an inlet and an outlet, means for feeding preheated feed water to said evaporator inlet, and a water separator connccted to said evaporator between said outlet and said inlet for recycling working medium through said evaporator; means for condensing at least some of the steam leaving said evaporator outlet on at least some of the preheated feed water for subsequent feeding with the preheated feed water to said evaporator inlet.

2. In a steam generator as set forth in claim 1 wherein said means for condensing is a mixer connected to said evaporator outlet for receiving steam therefrom, to said water separator to receive water therefrom and to said means for feeding preheated feed water.

3. In a steam generator as set forth in claim 2 wherein a bypass line is connected between said evaporator outlet and said mixer to bypass said water separator, said bypass line hav ing a control valve therein for controlling the flow of steam; and a line is connected between said water separator and mixer to convey water therebetween.

4. in a steam generator as set forth in claim 3 wherein said mixer has internal fittings therein for passage ol'the feed water and condensed steam jointly therethrough.

5. ln a steam generator as set forth in claim 4 wherein said fittings are perforated plates.

6. in a steam generator as set forth in claim 1 wherein said means for condensing includes a line between said means for feeding preheated feed water, a pair of branch lines connected between said line and said water separator, one of said branch lines being connected to said water separator above the water level therein and the other of said branch lines being connected to said water separator below the water level therein, and a distributor between said line and said branch lines for controlling the feed water between said branch lines.

7. In a steam generator as set forth in claim 6 wherein said water separator has an outlet connected to said evaporator inlet for feeding working medium thereto and wherein said means for condensing further includes a temperature sensitive element connected to said separator outlet and said distributor for controlling said distributor in response to the temperature in said separator outlet.

8 In a steam generator as set forth in claim I wherein a recirculating pump is disposed between said water separator and said evaporator inlet for recycling working medium to said evaporator.

9. A method of operating a forced flow steam generator constructed for subcritical pressure and having a water separator connected to and between an evaporator inlet and outlet therein comprising the steps of feeding preheated feed water to the evaporator inlet,

recycling working medium at part load from the water separator to the evaporator inlet,

condensing at least some of the steam leaving the evaporator at least in the top load range of the steam generator on at least some of the preheated feeding the condensed steam with the preheated feed water to the evaporator inlet.

10. A method as set forth in claim 9 wherein the temperature of the condensed working medium and feed water mixture is kept at a temperature of about l0 to 20 Centigrade below the saturated steam temperature of the steam leaving the evaporator.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3: 59 7 Dated y 3 97 Inventor(s) Ewald K. Burkle It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 9, "or" should be of Column line 39, after evaporator" insert including Signed and sealed this 26th day of March 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting-Officer Commissioner of Patents PC 1 USCOMM-DC scan-Pen 

1. In a steam generator having an evaporator parallel connected evaporator tubes defining and extending over the height of a combustion chamber, said evaporator further including an inlet and an outlet, means for feeding preheated feed water to said evaporator inlet, and a water separator connected to said evaporator between said outlet and said inlet for recycling working medium through said evaporator; means for condensing at least some of the steam leaving said evaporator outlet on at Least some of the preheated feed water for subsequent feeding with the preheated feed water to said evaporator inlet.
 2. In a steam generator as set forth in claim 1 wherein said means for condensing is a mixer connected to said evaporator outlet for receiving steam therefrom, to said water separator to receive water therefrom and to said means for feeding preheated feed water.
 3. In a steam generator as set forth in claim 2 wherein a bypass line is connected between said evaporator outlet and said mixer to bypass said water separator, said bypass line having a control valve therein for controlling the flow of steam; and a line is connected between said water separator and mixer to convey water therebetween.
 4. In a steam generator as set forth in claim 3 wherein said mixer has internal fittings therein for passage of the feed water and condensed steam jointly therethrough.
 5. In a steam generator as set forth in claim 4 wherein said fittings are perforated plates.
 6. In a steam generator as set forth in claim 1 wherein said means for condensing includes a line between said means for feeding preheated feed water, a pair of branch lines connected between said line and said water separator, one of said branch lines being connected to said water separator above the water level therein and the other of said branch lines being connected to said water separator below the water level therein, and a distributor between said line and said branch lines for controlling the feed water between said branch lines.
 7. In a steam generator as set forth in claim 6 wherein said water separator has an outlet connected to said evaporator inlet for feeding working medium thereto and wherein said means for condensing further includes a temperature sensitive element connected to said separator outlet and said distributor for controlling said distributor in response to the temperature in said separator outlet.
 8. In a steam generator as set forth in claim 1 wherein a recirculating pump is disposed between said water separator and said evaporator inlet for recycling working medium to said evaporator.
 9. A method of operating a forced flow steam generator constructed for subcritical pressure and having a water separator connected to and between an evaporator inlet and outlet therein comprising the steps of feeding preheated feed water to the evaporator inlet, recycling working medium at part load from the water separator to the evaporator inlet, condensing at least some of the steam leaving the evaporator at least in the top load range of the steam generator on at least some of the preheated feeding the condensed steam with the preheated feed water to the evaporator inlet.
 10. A method as set forth in claim 9 wherein the temperature of the condensed working medium and feed water mixture is kept at a temperature of about 10* to 20* Centigrade below the saturated steam temperature of the steam leaving the evaporator. 